On-line analysis of the chemical composition of a solute in a carrier medium (e.g., a solvent) combination has been accomplished using various known techniques, such as a gel permeation chromatograph (GPC) coupled to an ultraviolet (UV) spectrometer. Generally, the success of such a technique depends on how well the solutes can be differentiated from the carrier medium. This differentiation has been relatively easy in most cases, but there are cases in which the carrier medium seriously interferes with the on-line analysis of the solute, rendering a particular analysis technique less useful than desired.
For example, in flow-thru high pressure liquid chromatographic (HPLC) systems coupled to a Fourier transform infrared (FTIR) spectrometer, the chromatographic solvent often interferes with the detection of the solute. Therefore, various flow-thru cells (e.g., see C.C. Johnson and L.T. Taylor, Anal. chem., 56, 2642-2647 (1984)) as well as interfaces to eliminate the solvent prior to analysis (e.g., see C.M. Conroy, P.R. Griffiths, and K. Jinno, Anal. Chem., 57, 822-825 (1985)) have been designed. Eliminating low-boiling point solvents such as hexane is easy to accomplish, but not so with water or higher-boiling organic solvents. With high boiling point solvents such as trichlorobenzene (TCB), which is a common solvent in the analysis of polymers by gel permeation chromatography (GPC), the situation is one of the worst. Consequently, on-line polymer composition analysis in a TCB solvent system by GPC/FTIR has so far remained in the realm of concept only. Similarly, FTIR as a powerful and versatile analytical tool for HPLC, GPC and process analyzers has been limited in its application because of the solvent or process interference problems.
Ideally, to eliminate the solvent-solute interference problems, whether in a GPC experiment or in continuous process control, one would like to eliminate the solvent altogether. If a technique can eliminate TCB, then it readily may be used to eliminate substantially any other chromatographic or process solvent.
Overcoming the above mentioned problem in high-temperature (HT) GPC would benefit studies aimed at determining composition distributions (CD) in polymers (e.g.,L. Wild, T.R. Ryle, D.C. Knobeloch, and I.R. Peat, J. Polymer Science: Polymer Physics Edition, 20 441-455 (1982)). CD is the change in comonomer composition of polymer chains as a function of their molecular weight (MW). Most CD studies of crystalline or amorphous polymers have depended on large-scale fractionation or cross-fractionation, often followed by a slow solvent-stripping step to prepare the fractions for subsequent analytical measurements. Only in favorable cases, such as in semicrystalline polymers, where a relationship may be established between comonomer content and melting temperature has the need for an on-line composition detector not been essential. In contrast, amorphous polymers cannot benefit from such empirical relationships. Consequently, most separations have depended on solvent/non-solvent fractionations (e.g., H. Sato et al, Macromolecules, 19, 2613 (1986)). These operations are very tedious and time-intensive, and although the individual steps can be automated to reduce manpower requirements (D.L. Newhouse, R.G. Wheeler, and R.H. Waltz, U.S. Pat. No. 4,604,363 (1986)), the time-intensive nature of the analysis still remains a big hurdle to be overcome. Recent advances in HT-GPC-UV have been used for composition analysis (e.g., S. Mori and T. Suzuki, J. of Liquid Chrom., 4(10), 1685 (1981)), but the limitations imposed on the choice of solvent and the requirement that the polymer or solute must have a UV-active group restrict the applicability of this technique. Although FTIR is a less sensitive tool than UV, it is by far the more powerful structural tool because of its superior selectivity in terms of chemical species differentiation.
Another area where FTIR would be of tremendous importance is in the area of on-line analysis of liquid process streams, regardless if the stream is heterogeneous in nature or if the solutes are UV-inactive. The essential question is how to eliminate the solvent on-line for subsequent, automated on-line or off-line analytical measurements such as by FTIR.